Colloid means a state in which colloidal particles having a size of approximately several nm to several μm are dispersed in a medium. When an adequate condition is selected, colloidal particles are regularly disposed and form a structure called a “colloidal crystal”. There are two types of the colloidal crystal.
The first type is a crystal formed in a colloidal system without particular interactions between particles (hard sphere system) on the condition that the particle volume fraction is approximately 0.5 (the concentration=50% by volume) or more. This is similar to a phenomenon that when macroscopic spheres are stuffed into a limited space, the spheres are regularly disposed.
The second type is a crystal structure formed by electrostatic interactions acting between particles in a dispersion system of charged colloidal particles (a charged colloidal system). The crystal is for example formed in a colloidal system obtained by dispersing particles made of a polymer (polystyrene, polymethyl methacrylate, etc.) having a dissociable group on the surface thereof and silica particles (SiO2) in a polar medium such as water. Since electrostatic interactions extend to a long distance, the crystal can be formed even when the particle concentration is low (a distance between particles is long) and the particle volume fraction is approximately 0.001.
The present inventors have developed a means for controlling the formation of a colloidal crystal derived from a charged colloidal system (Patent Document 1). That is, the means is a method in which pyridine is added in a silica colloidal dispersion system, and by changing pH due to temperature changes using the temperature dependence of the degree of dissociation of pyridine, the number of surface charges on silica particles is changed to form a colloidal crystal. According to this method, colloidal crystals can be produced from a wide variety of charged colloidal systems without requiring special equipment and complicated steps.
The present inventors have further succeeded in producing a huge colloidal single crystal by developing this method (Patent Document 2). That is, this is a method for growing a colloidal crystal by putting a colloidal dispersion liquid, in which pyridine is added in a silica colloidal dispersion system, into a container in a state in which a colloidal crystal is not formed, setting one end of the container to the temperature at which a colloidal crystal is partially formed by heating, and growing a colloidal crystal by gradually expanding the range set to the temperature at which the colloidal crystal is formed. The colloidal crystal thus obtained is an extremely large single crystal and has fewer lattice defects and less unevenness.